必须单击上下文菜单两次

Question

我已向RichTextBox添加了上下文菜单，但是用户必须右键单击两次才能显示。我错过了什么？

Private Sub rtf_Notes_MouseUp(sender As Object, e As MouseEventArgs) Handles rtf_Notes.MouseUp

    If (e.Button = Windows.Forms.MouseButtons.Right) Then
        Dim contextMenu As ContextMenu = New System.Windows.Forms.ContextMenu()
        Dim menuItem_Font As New MenuItem("Selection Font...")
        AddHandler menuItem_Font.Click, AddressOf RightClick_Font
        Dim menuItem_Cut As New MenuItem("Cut")
        AddHandler menuItem_Cut.Click, AddressOf RightClick_Cut
        Dim menuItem_Copy As New MenuItem("Copy")
        AddHandler menuItem_Copy.Click, AddressOf RightClick_Copy
        Dim menuItem_Paste As New MenuItem("Paste")
        AddHandler menuItem_Paste.Click, AddressOf RightClick_Paste
        contextMenu.MenuItems.Add(menuItem_Font)
        contextMenu.MenuItems.Add("-")
        contextMenu.MenuItems.Add(menuItem_Cut)
        contextMenu.MenuItems.Add(menuItem_Copy)
        contextMenu.MenuItems.Add(menuItem_Paste)
        rtf_Notes.ContextMenu = contextMenu
    End If

End Sub